Applying SOLID and CLEAN CODE

[AmandaMuniz]

While refactoring the backend files two steps were made. The first one was to identify where exists the necessity to modify the code, to respect the solid principles, and made these changes. The second one was to apply clean code. Because of this, the following changes were made:

1. The file entity.py was empty, and therefore removed from the project;
2. The models Intent and Utter had a function called to_representation that was repeated and commented, therefore was deleted;
3. The file parser was refactored using clean code methods.

[AmandaMuniz]

This pull request resolves the following issue